1. Field of the Invention
The present invention relates to a handover method and apparatus using an X2 interface in the mobile communication system.
2. Description of the Related Art:
Long Term Evolution (LTE) is an evolution technology of the third Generation (3G) mobile communication system, and has several advantages, such as enhancing the capacity of cells and reducing the system delay.
FIG. 1 is a schematic diagram illustrating an architecture of a LTE system according to the related art.
Referring to FIG. 1, the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) of the LTE system typically includes radio resource management entities, such as evolved Node Bs (eNBs) and Home eNBs (HeNBs), and may further include HeNB GateWays (GWs). When the HeNB GWs are not included, the HeNBs may directly connect with Mobile Management Entities (MMEs) of the core network. When the HeNB GWs are included, the HeNBs connect with the MMEs through the HeNB GW. The MMEs are important network entities of the core network, and are responsible for functions such as, radio access bearer establishment and mobility management.
In the mobile communication system, in order to provide better services for particular users, as for a particular group, a Closed Subscriber Group (CSG) is usually needed to be formed with multiple radio resource management entities. For example, all users of a company or campus form a particular user group. A CSG is formed for this user group with multiple radio resource management entities to provide specialized access services.
The radio resource management entities of the LTE include eNB 101 and HeNB 1030. The radio resource management entities of the LTE may further include HeNB GW 103. Each eNB 101 is connected with each other through an X2 interface. Each eNB 101 is directly connected with MME 104 in a Core Network (CN) through an S1 interface. HeNB 102 may connect with HeNB GW 103 through the S1 interface, and then HeNB GW 103 may connect with MME 104 through the S1 interface. HeNB 102 may also directly connect with MME 104 through the S1 interface. When there is no HeNB GW 103 deployed in the system, HeNB 102 directly connects with MME 104 through S1 interface. Both eNB 101 and HeNB 102 may connect with multiple MMEs 104 in the CN.
In order to provide richer access services, the radio resource management entities of the LTE system shown in FIG. 1 usually include more types. For example, the HeNBs are classified into open HeNBs, hybrid HeNBs, and CSG HeNBs. The open HeNBs denote HeNBs which are not directed against any particular user group, and to which any User Entity (UE) may access. The CSG HeNBs denote HeNBs in the CSG user group, and only permit the access of UEs in the particular group served by the CSG HeNBs. The hybrid HeNBs denote the HeNBs which support the function of the CSG, permit the access of UEs in the particular user group served by the hybrid HeNBs, and also permit the access of the UEs in a general user group.
The UE may move among different HeNBs and between a HeNB and eNB. The movement of UE is implemented through S1 handover. The S1 handover represents handover using an S1 interface.
FIG. 2 is a schematic diagram illustrating an S1 handover procedure according to the related art. Suppose each of HeNBs connects with the MME through the HeNB GW.
Referring to FIG. 2, a source HeNB 217 sends a handover required message to a HeNB GW 219 in step 201a. How to send a measurement report to the source HeNB 217 from the UE 216 and how to initiate the handover by the source HeNB 217 is not introduced here.
In step 201b, the HeNB GW 219 sends the handover required message to the MME (220).
In step 202a, the MME 220 sends a handover request message to the HeNB GW 219, and in the step 202b, the HeNB GW 219 sends the handover request message to the target HeNB. The source HeNB 217 denotes the HeNB at which the UE 216 is originally located. The target HeNB 218 refers to the HeNB to which the UE performs handover.
In step 203, the target HeNB 218 allocates resources for the UE 216, and in step 203a, sends a handover request acknowledgement message to the HeNB GW 219. In step 203b, the HeNB GW 219 sends the handover request acknowledgement message to the MME 220.
In step 205a, the MME 220 sends a handover command message to the HeNB GW 219. In step 205b, The HeNB GW 219 sends the handover command message to the source HeNB 217.
In step 206, the source HeNB 217 sends a Radio Resource Control (RRC) connection re-configuration message to the UE 216.
In step 207, the UE 216 synchronizes to the target cell, and in step 208, sends the RRC connection re-configuration completion message to the target HeNB 218.
In step 209a, the target HeNB 218 sends a handover notify message to the HeNB GW 219. In step 209b, the HeNB GW 219 sends the handover notify message to the MME 220.
In step 210, the MME 220 sends a modify bearer request message to a Service-Gateway/Packet Data Network Gateway (S-GW/PDN GW) 225. The S-GW is mainly used for providing a user plane function. The PDN GW is mainly used for functions, e.g., charging and lawful interception. In step 211, the S-GW and PDN GW may be set in a same physical entity, or be two entities. This step omits the signaling interactions between the S-GW and PDN GW.
In step 212, the S-GW/PDN GW 225 sends a modify bearer response message to the MME 220.
In step 213, the UE 216 initiates a Tacking Area Update (TAU) process.
In step 213a, The MME 220 sends a UE context release command message to the HeNB GW 219. In step 213b, the HeNB GW 219 sends the UE context release command message to the source HeNB 217.
In step 214a, the source HeNB 217 sends the UE 216 context release completion message to the HeNB GW 219. In the 214b, the HeNB GW 219 sends the UE 216 context release completion message to the MME 220.
Although the above process may implement the handover procedure, taking the large number of HeNBs and the often handover of UE into consideration, if each handover is implemented through the S1 handover, the burden of the core network is heavily aggravated and the efficiency of the handover is reduced. Similarly, the above issues also exist when the UE moves between the HeNB and eNB, and between eNB and eNB.
In the related art as described above, movement of UE between HeNB and eNB is also achieved through S1 handover. From above analysis, it can be seen that movement of UE between HeNBs through S1 handover is achieved via the CN, which includes handover preparation and data forwarding. If each movement of UE between HeNBs is achieved through S1 handover, a very heavy load will be brought for the CN due to the huge number of HeNB. The handover efficiency may be reduced.
However, handover preparation and data forwarding in prior X2 handover process are not necessary to be performed through the MME. Thus, the X2 handover process may be applied for the movement of UE between HeNBs.
FIGS. 3A, 3B, and 3C are a connection schematic diagram illustrating applying X2 handover to an HeNB according to the related art. FIG. 3A is a connection schematic diagram illustrating movement (300) of a UE between HeNBs 305a and 305b through X2 handover. FIG. 3B is a connection schematic diagram illustrating movement (310) of a UE from HeNB 315 to eNB 320 through X2 handover. FIG. 3C is a connection schematic diagram illustrating movement (325) of UE from eNB 330 to HeNB 335 through X2 handover.
Referring to FIGS. 3A to 3C, since the HeNB GW possesses Non-Access-Stratum (NAS) Node Selection Function (NNSF), the HeNB GW will select a serving MME for a UE under the HeNB when the HeNB accesses the MME through the HeNB GW. However, the HeNB doesn't learn of the MME selected by the HeNB GW for the UE. Accordingly, when executing the X2 handover, the source HeNB cannot inform the target HeNB about the serving MME of the UE. Subsequently, the target HeNB, or the eNB, or the target HeNB GW will not learn to which MME the path switch request message should be sent. The handover process will be unsuccessful if the message has been sent to other MME instead of the MME initially accessed by the UE.